1. Field Of The Invention
This invention relates generally to a process for preparing aqueous compositions including polymeric latex and titanium dioxide pigment, and more particularly to aqueous compositions prepared for use as coatings compositions.
2. Background Of The Invention
Aqueous dispersions of polymeric latex are used to prepare a tremendous variety of commercially important products, including paints, coatings, primers, mastics, caulks, binders for non-woven materials, and adhesives. Frequently, the polymeric latex is included to form a continuous phase as the water is lost in order to bind the product together, and to contribute important physical properties.
Titanium dioxide has been for many years the pigment of choice for conferring opacity to plastic sheets and films, and particularly to coatings formed from coating compositions and paints. In paints titanium dioxide is typically the most expensive component of the formulation. It has been an ongoing objective of the coatings industry to achieve the desired degree of opacity in a coating while at the same time using as little titanium dioxide pigment as possible. One way in which this is done is by employing titanium dioxide which has an optimal average particle size and particle size distribution for scattering light. Another way of making efficient use of the titanium dioxide employed is by dispersing this pigment as well as possible.
Agglomerated or aggregated titanium dioxide pigment particles make less than their full potential contribution to the performance of the coatings, such as, for example, with regard to coating opacity and colorant potential.
The opacifying capability or hiding power of a paint or coating is a function of the spacing of the titanium pigment particles in the dried coating. The light scattering characteristics of titanium dioxide particles are well known. The average size and size distribution of titanium dioxide particles used for opacifying purposes has been highly optimized by the titanium dioxide manufacturers for maximum scattering. Maximum light scattering occurs when the titanium dioxide pigment particles have a diameter of from about 200 to about 250 nanometers and are spaced far apart from each other, on the order of a few particle diameters, so that there is minimal interference between the light scattering of neighboring particles.
In practice, however, for example in the formulation of paints, it is widely recognized that when enough titanium dioxide particles have been dispersed into the polymeric coating vehicle to yield films of acceptable opacity, the level of opacity which is achieved is significantly less than would be theoretically predicted from the light scattering potential of a single titanium dioxide particle multiplied by the total number of particles in the path of light passing through the film.
A number of factors have been identified which partially account for the diminution of opacity from the theoretical predictions. If two or more titanium dioxide particles are in actual contact with each other, or even if they are closer together than the optimum spacing distance, the particles will behave like a single aggregated larger particle and result in reduced light scattering. This occurs if the titanium dioxide particles are not adequately deaggregated during the dispersion process. However, even if the titanium dioxide particles are fully deaggregated in the dispersion process, a random distribution of particles will not provide the maximum scattering achievable in theory if the particles were optimally distributed.
In a related phenomenon, referred to as "crowding," titanium dioxide pigment particles are forced to be nearer to each other than the desired optimum merely by the lack of available space. This lack of available space may be caused by the space taken up by the other coating fillers and extenders which are of a comparable size to, or which are larger than, the pigment particles. In the case of polymeric binders, which are particulate in nature prior to film formation, such as, for example, latex or emulsion polymers, and in the case of nonaqueous dispersion polymers, the binder polymer particles themselves can crowd pigment particles, especially if they are of comparable or larger in size than the titanium dioxide particles.
There is a continuing need to improve the effective utilization of inorganic materials such as titanium dioxide in aqueous coating compositions and thereby to improve the opacity and other performance properties of coating compositions.